The present invention relates to photodiodes and, more particularly, to a novel method for fabricating Schottky photodiodes.
Schottky photodiodes have received extensive use in imaging applications, particularly imaging for medical purposes such as infrared imaging, ultraviolet imaging, x-ray imaging and the like. Basically, light energy or similar electromagnetic energy, incident upon a photodiode, or an array of photodiodes, will create an electrical signal, or signals, which can be processed to produce an image which can be displayed by means of a cathode ray tube or the like.
Conventional Schottky photodiodes are fabricated by depositing a base or bottom electrode on a principal substrate surface. A layer of doped amorphous silicon (a-Si) follows. A layer of intrinsic silicon (i-Si) is formed on the a-Si layer and a first layer of passivation material such as silicon nitride (SiN) or the like is formed over the i-Si layer. This multilayer "sandwich" is then patterned and etched with a first mask by known photolithographic techniques to form a diode island. A second layer of passivation material, such as SiN, is deposited over the diode island; both passivation layers are then patterned and etched with a second mask to form a via opening which exposes a portion of the i-Si layer top surface. A top electrode, or Schottky contact, is disposed on the second passivation layer and on the exposed i-Si layer portion through the via opening. That area where the Schottky material electrically contacts the exposed i-Si portion is referred to as the active area. Problems can arise with this fabrication method because of pattern transfer errors between the photoresist mask and the second passivation layer in the second masking operation, which can cause the via opening to be misaligned with the diode island. If the via opening is misaligned to the extent that it extends beyond the edge of the i-Si layer top surface and also exposes the underlying doped a-Si layer or the base electrode, the subsequently deposited Schottky contact metallization will short-circuit the diode and the device will not function. To minimize misalignment errors and etching errors, the via opening formed in the second mask operation is made smaller than the top surface of the diode island; however, this reduces the size of the active area of the photodiode and adversely affects some photodiode performance characteristics, such as reduced signal-to-noise operation. The reduction of active area is not very significant with larger photodiodes, but when high packing density design constraints require small photodiodes, the loss of active area becomes more critical. Additionally, current methods of fabricating Schottky photodiodes typically employ at least two critical masking and etching steps; besides reducing the photodiode's active area as explained above, the multiple masking steps also reduce throughput and yield.
It is accordingly a primary object of the present invention to provide a novel method for fabricating Schottky photodiodes which is not subject to the foregoing disadvantages.
It is another object of the present invention to provide a novel method for fabricating Schottky photodiodes which maximizes the active area.
It is a further object of the present invention to provide a novel method for fabricating Schottky photodiodes which requires a minimum number of masking steps.
These and other objects of the invention, together with features and advantages thereof, will become apparent from the following detailed specification when read with the accompanying drawings in which like references numerals refer to like elements.